The present invention relates to a printer for forming an image on a continuous recording sheet utilizing an electrophotographic system.
There are conventionally known image forming apparatuses utilizing a so-called electrophotographic transfer system, such as an electronic copying machine, in which the surface of a photoconductive drum is exposed to light to form a latent image on the drum surface. Toner is applied to the latent image for development and the developed image is transferred onto a recording sheet and fixed by a fixing unit.
Another such apparatus is the laser beam printer which provides a hard copy of image information by scanning and exposing a charged photoconductive drum, to a laser beams modulated based on image information such as figures, characters and the like, using the copy process of the above mentioned electrophotographic transfer system.
The laser beam printer is very useful, because it can be widely used in such a manner that it draws figures of information received by an image reading unit (image scanner) or used as an output terminal of a facsimile. Further, the laser beam printer outputs information at a high speed.
In general, a conventional laser beam printer comprises an existing electronic copying machine as a base unit and uses a sheet cut to a predetermined size, and a heat roll fixing unit, which includes a pair of fixing rollers with a heat roller heated to a high temperature and a backup roller pressed against the heat roller. An unfixed toner image is transferred onto a recording sheet, and the recording sheet is thereafter caused to pass between the fixing rollers to be heated and pressed, whereby the toner is melted and adheres to the recording sheet.
The electrophotographic system is such that the rotation of a photoconductive drum causes an exposed portion of the drum to reach a transfer unit. A toner image is then transferred onto the recording sheet which is fed at a speed identical with the peripheral speed of the photoconductive drum at the transfer unit. Thus it is impossible to form images by intermittently interrupting the process due to the structure of the process.
Therefore, the laser beam printer is provided with a memory capable of storing at least one page of information. When the one page of information is inputted therein, the printer outputs the information in one lot.
It can be considered, of course, to use this laser beam printer as an output terminal of a computer. In this case, however, it is preferable to use a continuous recording sheet (a fan-fold sheet having feed holes and folded along perforated tear lines enabling the sheet to be simply torn off, hereinafter, referred to as a continuous sheet) which has been used in a conventional line printer. The use of this continuous sheet has many advantages. The output data can be observed sequentially as it is outputted. The data can be easily arranged because it is continuously outputted and a number of pages can be supplied without using a dedicated stacker. The feed of the sheet (control of printing positions) can be correctly and easily adjusted because it is fed by the feed holes, and the like.
When a continuous sheet is used to a laser beam printer using heat roll fixing system, the length of a recording sheet feed path from the transfer unit to the fixing position of a fixing unit must be set to substantially the same distance as the distance between perforated tear lines of a continuous sheet. This prevents the disadvantage that when the laser beam printer stops (for wait or because an operation is completed), a page being subjected to a fixing action is stopped between a pair of fixing rollers and a page of the continuous sheet on which a toner image is in the process of being fixed remains nipped between the pair of the fixing rollers.
More specifically, since the continuous sheet is cut off along the perforated tear lines for use, no image must be formed within a predetermined region in the vicinity of the perforated tear lines. Thus, in a laser beam printer where images are formed for each page, the vicinity of the tear lines is arranged to be stopped in a state that corresponds to the transfer unit. Consequently, when the length of the recording sheet feed path from the transfer unit to the fixing position is set to substantially the same distance as between perforated tear lines of the continuous sheet, the above disadvantage can be avoided, because the vicinity of the tear line where no image is formed is caused to correspond to the position of the fixing unit where the fixing action is effected, when the laser beam printer stops.
As a result, however, a problem arises in that continuous sheets having a different distance between perforated tear lines (i.e., a different length of a page) cannot be interchangeably used in the same laser beam printer. That is, continuous sheets having different intervals between perforated tear lines cannot be used.